This invention relates to the application of a continuous impervious polymer coating to a cementitious wall and more particularly is directed to a method of applying a polyethylene coating to the interior of a hollow article especially a cylindrical body such as cement pipe and cement lined iron pipe and to the coated pipe resulting therefrom.
It is known that plastic coatings, including coatings of polyethylene and other olefin polymer or copolymer materials, provide reduced friction coefficients and protection against corrosion and or erosion when applied to metal and or cement surfaces. This applies particularly to pipes made of metal or cement which heretofore have benefited from protective internal coatings of bitumen or coaltar epoxy as shown in U.S. Pat. No. 4,474,134, the disclosure of which is included herein by reference, to resist the destructive properties of liquids such as potable water, sewage or other substances transported therethrough. Indeed some ferrous pipes utilize an interior layer of cement as a protective coating to resist corrosion. Apparatus for producing cement lined pipe and other vessels is shown in U.S. Pat. Nos. 4,414,918; 3,563,791 and 2,598,972, the details of which are included herein by reference.
While the advantages of a plastic interior pipe coating have been known, the application of materials such as polyolefins has required expensive surface preparation and even so the coating tends to delaminate from the pipe surface. Prior techniques for applying polyolefins such as polyethylene to the interior surface of pipe are described in U.S. Pat. Nos. 3,348,995; 4,007,298; 4,254,165 and 4,407,893, the details of each are also incorporated herein by reference. While the following description deals largely with polyethylene, it is to be understood that other known thermo plastic coating material having desirable coating characteristics, such as polyurethane and polyvinylchloride may also be suitable provided they are chemically stable, compatible with the ingredients of cement and otherwise suitable for rotational molding techniques.